Unknown user setting in Central Email

How is an "Unknown user" or "No User Available" handled in Central Email?

Is there a "CatchAll" setting somewhere so I can designate all incoming emails that don't have legitimate recipients go to?

  • Open a gmail or yahoo email up and send a fake user email to your domain and see what happens. 

     

    Also and because I am curious on the outcome here, are you hosting your own email servers internally or do you have Office 365 setup with a site relay?